Surface ferromagnetism and exchange bias in vacuum annealed Co3-y Zny O4 films

Article abstract of DOI:10.1063/1.2836792

We report on the structural and the magnetic properties of spin coated Co3-y Zny O4 thin films. The microstructure of high vacuum annealed films exhibit a crystalline Zn:CoO core with a disordered oxygen deficient amorphous Co:ZnO surface. Weak ferromagnetism (m～15 m μB Co) and an exchange bias (HE ～500 Oe) are observed at 10 K despite the absence of any Co metal clusters. HE is smaller for samples with smaller Zn fractions (for Co3 O4 -HV, HE ～250 Oe). Adding Zn significantly enhances HE. The exchange anisotropy in an antiferromagnetic material due to surface disordered weak ferromagnetism and its overall implications to dilute magnetic semiconducting oxides are discussed.

Full text of DOI:10.1063/1.2836792

Surface ferromagnetism and exchange bias in vacuum annealed Co 3 − y Zn y O 4 films
C. Sudakar, P. Kharel, G. Lawes, R. Suryanarayanan, R. Naik, and V. M. Naik
Citation: Applied Physics Letters 92, 062501 (2008); doi: 10.1063/1.2836792
View online: http://dx.doi.org/10.1063/1.2836792
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APPLIED PHYSICS LETTERS 92, 062501 ͑2008͒
Surface ferromagnetism and exchange bias in vacuum annealed
Co3−yZnyO4 films
C. Sudakar,a͒ P. Kharel, G. Lawes, R. Suryanarayanan,b͒ and R. Naik
Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201, USA
V. M. Naik
Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, Michigan 48128, USA
͑Received 8 October 2007; accepted 27 December 2007; published online 11 February 2008͒
We report on the structural and the magnetic properties of spin coated Co3−yZnyO4 thin films. The
microstructure of high vacuum annealed films exhibit a crystalline Zn:CoO core with a disordered
oxygen deficient amorphous Co:ZnO surface. Weak ferromagnetism ͑mϳ15 m␮B/Co͒ and an
exchange bias ͑HEϳ500 Oe͒ are observed at 10 K despite the absence of any Co metal clusters. HE
is smaller for samples with smaller Zn fractions ͑for Co3O4-HV, HEϳ250 Oe͒. Adding Zn
significantly enhances HE. The exchange anisotropy in an antiferromagnetic material due to surface
disordered weak ferromagnetism and its overall implications to dilute magnetic semiconducting
oxides are discussed. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2836792͔
Measurements on oxidized fine Co nanoparticles typi-
cally exhibit displaced hysteresis loops when measured on
field cooling, arising from an exchange bias at the interface
between the ferromagnetic ͑FM͒ core and antiferromagnetic
͑AFM͒ CoO shells.1 The study of FM-AFM exchange inter-
actions in fine particles is relevant for applications such as
read heads and nonvolatile memory.2 Most current models
for the exchange bias consider the role of uncompensated
AFM spins at the FM/AFM interface.3–5 These uncompen-
sated spins lead to a net magnetization, which is assumed to
couple to the neighboring FM phase. This coupling results in
a larger FM coercive field when the components are antipar-
allel, but a smaller coercive field when the components are
parallel. Very fine AFM particles can also exhibit weak FM
as a result of unbalanced magnetic moments of the two mag-
netic sublattices due to finite size effects leading to uncom-
pensated surface spins.6
We report on the experimental observation of weak FM
and exchange bias in vacuum annealed Co3−yZnyO4 ͑y=0,
0.25, and 1͒ thin films, and discuss the effect on Zn in en-
hancing the exchange bias. Additionally, Co doped ZnO is an
important diluted magnetic semiconducting oxide ͑DMSO͒
material, which is being investigated as a potential candidate
for spintronics applications.7 The samples in our studies can
be considered as the Co rich limit of Co doped ZnO DMSO.8
We discuss our results in the context of understanding the
origin of ferromagnetism in these systems. The exchange
bias has been studied extensively on FM Co metal with AFM
oxide surface layers, including Co–CoO bilayers9–11 and
core-shell nanoparticles.11,12 However, in contrast to these
earlier investigations, our results show an exchange biased
Co1−yZnyO phase by high vacuum annealing of AFM
Co3−yZnyO4, without intentionally including any Co metal
phase and starting from a homogeneous AFM sample.
Vacuum annealing Co3−yZnyO4 produces a Co1−yZnyO core
structure with a disordered surface component, which we
argue leads to the development of weak FM.
Co3−yZnyO4, thin films with thickness of ϳ0.5 to 1 ␮m
with y=0, 0.25, and 1 were synthesized by metal organic
decomposition using a spin coating technique.8 The final an-
nealing was done at 600 °C in air for 1 h. The high-vacuum
͑HV͒ annealed films were obtained by annealing at 550 °C
for 1 h at a pressure of 10−5–10−6 Torr. We prepared three
different compositions Co3O4, Co2.75Zn0.25O4, and Co2ZnO4,
which are labeled as CO, CZ25O, and CZO, respectively, in
the following text. These labels are appended with “-A” and
“-HV” to indicate air and vacuum annealed samples. The
x-ray diffraction patterns ͑XRD͒ of air annealed films of
Co3−yZnyO4 showed all reflections corresponding to the spi-
nel structure of Co3O4.13. The XRD patterns for y=0.25
͑CZ25O-A͒ and y=1 ͑CZO-A͒ show that this spinel structure
is not altered by Zn substitution. Energy dispersive x-ray
spectroscopy analyses, performed at several locations, show
a uniform chemical composition and the expected cationic
ratio, suggesting that the films are homogeneous. The XRD
data indicate that a CoO phase is formed in CO-HV thin
films. Similar XRD patterns are observed for CZ25O-HV
and CZO-HV samples, which imply that the Co3−yZnyO4 spi-
nel phase reduces to a Co1−yZnyO ͑NaCl structure͒ phase.
Additional low intensity x-ray reflections in the CZO-HV
thin films suggest that disordered Zn1−xCoxO ͑wurtzite͒ may
also be present. Most importantly, there is no evidence for
Co metal clusters ͑hcp or fcc type͒ in any of the XRD
patterns.
Because XRD is insensitive to small precipitates of a
secondary phase, we also used transmission electron micros-
copy ͑TEM͒ to characterize the samples. All our films are
comprised of particles having similar shapes and sizes
͑ϳ75 nm͒, which allows us to neglect any microstructure
dependent changes in magnetic properties over a wide range
of composition. Figure 1 show TEM micrographs of air and
HV annealed CZO and CZ25O films. We do not observe any
substantial microstructural changes for the CZ25O and CO
thin films on annealing. However, for the Co2ZnO4 sample,
we observe some secondary phase at the boundary of the
crystalline particles. The high-resolution TEM image of one
such particle shows a poorly crystallized phase segregated on
the CoO particles. Selected area electron diffraction ͑SAED͒
patterns also confirm that Co3O4 reduces to CoO, while
a͒
Author to whom correspondence should be addressed. Electronic mail:
csudakar@gmail.com.
b͒
Permanent address: ICMMO, LPCES, UMR8182, Université Paris-Sud,
91405 Orsay, France.
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062501-2
Sudakar et al.
Appl. Phys. Lett. 92, 062501 ͑2008͒
gesting that a large number of oxygen defects are developed
on the surface, consistent with the high resolution transmis-
sion electron microscopy ͑HRTEM͒ images.
Based on these structural characterizations, we propose
that the single phase Co3−yZnyO4 films undergo a structural
decomposition to crystalline Zn:CoO ͑rocksalt structure͒ and
amorphous Co:ZnO ͑wurtzite structure͒ on HV annealing.
The amorphous Co:ZnO phase is located at the surface of the
particles and have a high concentration of oxygen vacancy
defects. The CO-HV particles consist of a CoO core with a
disordered component at the surface, while the CZO-HV par-
ticles have a Co1−xZnxO core with surface disordered com-
ponent surrounded by poorly crystallized Zn1−xCoxO phase.
The magnetic moment at 300 K, measured using a QD
MPMS, is ϳ1.5ϫ10−3␮B/Co for the air annealed samples,
which increases to ϳ7ϫ10−3␮B/Co on HV annealing ͓Fig.
2͑a͔͒. The moment per Co is approximately the same for all
three compositions. The CO-HV sample shows a coercivity
͑HC͒ of ϳ100 Oe at 300 K. This HC increases to 500 Oe
with increasing Zn content. We attribute the FM to structur-
ally disordered surface layers of the ͑Zn doped͒ CoO nano-
particles. In this framework, the large room temperature HC
arises from exchange anisotropy ͑as discussed below͒, and
the increase in HC on Zn doping is attributed to an increase
in the magnetocrystalline anisotropy of Co1−xZnxO.15 Figure
2͑b͒ shows the zero-field cooled ͑ZFC͒ and field cooled ͑FC͒
magnetizations for these samples. In the air annealed thin
films, the ZFC-FC curves show a slight irreversibility below
350 K. The irreversibility persists up to 300 K in Co3O4,
well above the Néel temperature ͑TN͒ of 30 K, pointing to
the existence of nanoscale FM clusters at higher tempera-
tures. With increasing Zn content this irreversibility becomes
smaller at higher temperatures in the spinel films. The HV
annealed films all show significant ZFC-FC irreversibility,
with a much larger separation between the ZFC and FC
curves. The curves also show the CoO AFM transition
around 295 K for the CO-HV sample. From the 300 K M-H
curves, we find that CO-HV has HC of ϳ100 Oe, while the
CZ25O-HV and CZO-HV thin films have HC of ϳ200 and
ϳ500 Oe, respectively. These coercivities increase linearly
with decreasing temperature. CZO-HV has a maximum HC
ϳ1200 Oe at 10 K, while CO-HV has a HCϳ500 Oe at
10 K.
FIG. 1. TEM images of ͑a͒ CZO-A and ͑b͒ CZO-HV. The insets show the
pattern with the simulated diffraction pattern for Co2ZnO4 spinel phase in
͑a͒ and CoO ͑solid lines͒ and Co ͑dotted lines͒ phase in ͑b͒. Inset on the
right panel in ͑b͒ show the HRTEM images of particle surface. The CO-HV
particle has a crystalline CoO core with a disordered surface component.
The CZO-HV particle has a crystalline Co1−xZnxO core surrounded by amor-
phous ͑or weakly crystalline͒ Zn1−xCoxO. ͑c͒ Co 2p and ͑d͒ O 1s XPS
spectra of CZO-A and CZO-HV.
Co2ZnO4 decomposes to mixed Zn1−xCoxO ͑wurtzite͒ amor-
phous and Co1−yZnyO ͑rocksalt͒ crystalline phases. Most im-
portantly, no metal cluster related reflections were observed
in these samples, which precludes the presence of FM Co
metal clusters.
To rule out the presence of Co clusters, we characterized
the valence state of Co using x-ray photoelectron spectros-
copy ͑XPS͒. Figures 1͑c͒ and 1͑d͒ show a typical Co 2p and
O 1s XPS core spectra for the CZO-A and CZO-HV samples.
We did not observe any spectral features corresponding to
metallic Co ͑ϳ778 eV͒ in the HV annealed thin films. For
the CZO-A and CZO-HV, the main oxygen peak due to O2−
in the lattice is at ϳ529.8 and 529.4 eV, respectively. A
smaller secondary peak is seen in CZO-A at ϳ531.1 eV.
This is close to reported values for surface chemisorbed oxy-
gen or undercoordinated lattice oxygen.14 This secondary
peak is more prominent for the HV annealed samples, sug-
In order to carefully explore the nature of the FM in
these samples, we measured hysteresis loops at 10 K after
cooling from 350 K in 0 and 5 T fields to look for an ex-
change coupling between the FM and AFM components
FIG. 2. ͑Color online͒ ͑a͒ Room tem-
perature M-H loops for the air ͑blue
closed symbol͒ and HV ͑red open
symbol͒ thin films of CO, CZ25O, and
CZO. ͑b͒ Magnetization vs tempera-
ture curves under ZFC ͑open symbols͒
and FC ͑filled symbols͒ conditions
with an applied field of 500 Oe of air
͑blue triangle͒ and vacuum ͑red
square͒ annealed samples. ͑c͒ ZFC and
FC ͑with 5 T͒ M-H curves measured
at 10 K for vacuum annealed CO,
CZ25O, and CZO thin films.
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062501-3
Sudakar et al.
Appl. Phys. Lett. 92, 062501 ͑2008͒
This proposal is consistent with previous observations of
large magnetizations in AFM nanoparticles in granular CoO
films18 and in NiO.19,20
In summary, we observe history dependent magnetiza-
tion in vacuum annealed spin coated Co3O4 films, which we
attribute to strong exchange bias coupling. We argue that the
magnetic properties can be understood from the weak FM
produced by uncompensated surface spins which are coupled
to the intrinsic AFM order of the sample. Adding Zn to the
system significantly enhances the exchange coupling, but
does not substantially modify the magnetic moment per Co.
This increase in the exchange bias is not likely to arise from
interfacial effects, but may be due to the formation of do-
main states.21 Furthermore, despite extensive sample charac-
terization, no metallic Co clusters are observed when the
Co3−yZnyO4 films are reduced. While this does not absolutely
preclude the formation of Co metal clusters in HV annealed
lightly doped Co:ZnO films,8 the absence of metal clusters in
our Co rich samples at least suggests that such clusters are
not a generic feature of transition metal doped oxide films.
Additionally, these results offer an alternate approach to de-
veloping exchange biased systems beyond preparing FM/
AFM nanostructures or oxidizing ferromagnetic metals. Our
measurements suggest that one can prepare an exchanged
biased system starting from a purely AFM material by intro-
ducing weak FM by vacuum annealing.
FIG. 3. ͑a͒ Temperature dependence of the exchange bias for CO-HV,
CZ25O-HV, and CZO-HV. ͑b͒ Thermoremanent magnetization ͑TRM͒ mea-
sured under zero field during warm up after field cooling under 5 T for HV
annealed samples.
͓Fig. 2͑c͔͒. The hysteresis loop measured after cooling in a
+5 T field shows a negative field shift ͑HE͒ of ϳ500 Oe for
CZO-HV films, with smaller shifts for the samples with
smaller Zn fractions. When field cooled under −5 T ͑not
shown͒ the curves show a positive field shift with approxi-
mately the same magnitude. We ascribe these shifts to an
exchange bias coupling.11 While CO-HV samples also show
exchange bias effects, the shifts are much smaller
͑ϳ250 Oe͒ than CZO-HV films. The temperature depen-
dence of HE ͓Fig. 3͑a͔͒ exhibit a monotonic decrease with
increasing temperature and drops to zero near 300 K, right
around TN. This loop shift is accompanied by an increase in
coercivity, confirming the presence of exchange bias
coupling.11
To investigate the temperature dependence of the mag-
netic pinning, we measured the decay of the thermoremanent
moment ͑TRM͒ on warming ͓Fig. 3͑b͔͒. For these measure-
ments, the samples were cooled from to 350 to 10 K with an
in-plane field of 5 T, and the magnetization was measured at
zero field on warming. A remanent magnetization is pro-
duced by mechanisms that pin the FM moment, including
exchange bias coupling. The TRM drops by 40% but does
not vanish completely at TN of CoO suggesting that a large
fraction of the FM spins are exchange coupled to the uncom-
pensated AFM spins. The nonzero TRM above 300 K sug-
gests that there may be some residual weak FM at higher
temperatures. The TRM temperature decay curves for the
three samples also show qualitatively different behavior. For
CZO-HV films the residual moment between 50 and 225 K
is almost constant, showing only a slight decrease on warm-
ing. Conversely, the TRM for the other two samples
͑CZ25O-HV and CO-HV͒ show much stronger temperature
dependence. None of the TRM traces show any substantial
increase at low temperatures, which indicates absence of
weakly coupled spins or a possible “frozen” spin-glass
state.16
We gratefully acknowledge support for this project pro-
vided by the Institute for Manufacturing Research at Wayne
State University.
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